In the philosophy of perception, the argument of illusion is an argument in favor of the thesis that we never perceive anything but sense data. It presents itself as a critique of direct realism and relies on the most common natural illusions.
(A pencil partially immersed in a glass of water offers a simple example of a natural illusion. )
The argument of illusion was initially defended by Alfred Jules Ayer and has been debated extensively in the contexts of philosophy of mind and philosophy of science.
Exemplary case of the Descartes stick
The case of the stick half immersed in water, initially advanced by Descartes, is a classic example of a natural illusion: I have a stick that seems straight to me but when I hold it under water, it seems to bend and deform. I know that the stick is straight and that its apparent flexibility is an effect related to the perception that I have of him through the water, but, in spite of everything, I can not change the mental image that I have of stick as it is curved. Given the fact that the stick is not curved, its appearance can be described as an illusion, but this illusion is not fundamentally different from the normal perception of the object. In both cases, rather than perceiving the stick directly, we perceive a stick image consisting of a set of “sense data”. This mental representation tells us nothing about the real properties of the stick, which remain inaccessible to our senses.
Indiscernibility of truthful perceptual experience and illusion
The argument of the illusion rests on the thesis that a truthful experiment and an illusion can be similar in all points, in the sense that the subject can not know in which of the two states it is. From this it follows that truthful and illusory experiences are the same type of experience having the same type of object. Having admitted that the illusory experiences have for their object sense-data, the argument concludes that all perceptive experience, truthful as well as illusory, has for its object a set of sense data. Our senses would not put us in touch with the world as it exists in itself, but only with representations or mental contents.
Criticism of the argument
Many critics of the argument from illusion have been formulated. For proponents of the disjunctive theory of perception (Hinton, John McDowell), the fact that one can not distinguish from the inside between a truthful experience and an illusion does not imply that the two experiences constitute the same mental state. Moreover, we do not think that they should accept the idea that the object of an illusion is an authentic object.